1. Field of the Invention
The present invention relates to a total organic carbon measuring device (also referred to as a “TOC meter”) for measuring the total organic carbon (TOC) content, total carbon (TC) content, or inorganic carbon (IC) content of sample water, for example, a total organic carbon measuring device in which organic matter is separated from water containing few impurities, called pure water or ultrapure water, by a carbon dioxide separation unit to assay the TOC concentration of the water based on conductivity.
2. Description of the Related Art
For the purpose of management of water containing few impurities, such as water for manufacturing drugs, process water for semiconductor manufacturing, cooling water, boiler water, or tap water, the organic matter (TOC) content of a sample of such water is measured.
As a TOC measuring device, a TOC meter comprising a total carbon combustion unit using an oxidation catalyst is widely used. The TOC meter measures TOC by converting TOC contained in sample water to CO2 gas and measuring the CO2 concentration in the gas phase by a nondispersive infrared analyzer.
On the other hand, a device that measures the TOC concentration of sample water while keeping the sample water in the liquid phase has also been developed. In the case of such a device that measures sample water while keeping it in the liquid phase, organic matter contained in sample water is converted to carbon dioxide by an oxidation reactor. The sample water is kept in the liquid phase and is allowed to flow through a sample water channel. The sample water channel is in contact with a measurement water channel, through which measurement water flows, with a gas permeable membrane being interposed therebetween, and therefore carbon dioxide contained in the sample water is transferred to measurement water. The measurement water containing carbon dioxide transferred from the sample water is sent to a conductivity meter to measure the conductivity thereof. The carbon dioxide concentration of the sample water can be determined from the measured conductivity of the measurement water, and a previously-prepared calibration curve representing the relationship between the conductivity of measurement water and the carbon dioxide concentration of sample water (see Patent Document 1).
The present invention is directed to such a device that measures the TOC concentration of sample water while keeping the sample water in the liquid phase.
In the measurement of TOC, a measuring device needs to have a blank value of its own, that is, a system blank value. The system blank value of a TOC meter is a signal obtained by measuring, as a sample, pure water whose organic carbon content is infinitely close to zero, and is used as a reference for measurement by the TOC meter. Such a system blank value is essential to quantitative measurement of very low TOC levels of pure water and the like.
In pure water that is in contact with air, CO2 contained in air is dissolved. Therefore, it is difficult to obtain pure water whose carbon content is infinitely close to zero when the pure water is exposed to the atmosphere. In the case of a conventional TOC meter that comprises a total carbon combustion unit using an oxidation catalyst and measures TOC by converting TOC contained in sample water to CO2 gas and measuring the CO2 concentration in the gas phase, pure water is vaporized while TOC contained in the pure water is decomposed to CO2 gas by oxidation and removed, and then remaining water vapor is condensed to recover the pure water. The cycle of vaporization, oxidative decomposition, and recovery is repeated in this device to obtain pure water whose organic carbon content is infinitely close to zero. The conventional TOC meter uses the measured value of the thus obtained pure water as a system blank value. Patent Document 1: WO2008/047405
The present invention is directed to a TOC measuring device that is not provided with, as a channel for measurement, a channel for performing vaporization and oxidative decomposition of sample water at the same time and then condensing the remaining water vapor. In order to provide a channel for preparing blank water used to obtain a system blank value, it is necessary to provide not only a channel different from a channel for measurement but also a container intended for exclusive use as a pure water trap. Even when such a channel for preparing blank water is provided, it takes a long time to obtain pure water whose organic carbon content is infinitely close to zero by repeated cycles of vaporization and oxidative decomposition of sample water and condensation of water vapor.